1. Field of the Invention
The present invention relates generally to the field of telecommunications and more specifically a new efficient variable rate coding (VRC) of voice over asynchronous transmission Mode (ATM) using enhancements to the utilization of ATM adaptation layer of type 2 (AAL2). Possible ATM transport, trunking and access applications encompass both Circuit Emulation Services (CES) and Local Loop Emulation Services (LLES).
2. Discussion of the Background Art
Speech compression or coding with many standardized techniques reduces the bandwidth utilization of voice transmission and storage. Silence suppression (SS) improves average compression rate and is layered over the conventional constant rate speech coders. SS popularity is evident by the recent ITU (International Telecom Union) standardization of 8 kb/s G.729 coding in combination with SS under G.729-D. Application of SS in combination with other standard compression techniques is brought to VoIP (Voice over Internet Protocol) and VTOA (Voice Telephony over ATM) products. IETF, ITU, and ATM Forum have had related standard activities. Most notably, ATM Forum standardized ATM Adaptation Layer of type 2 (AAL2) provides an accepted scheme for ATM system integration and interoperation.
Silence suppression can, however, be considered the first step in VRC. Note that VRC as utilized here, and as exemplified later in this disclosure, refers to a specific variable voice coding (narrowband or wideband) whereby the rate adaptation is based mainly on variations in voice information content while maintaining certain subjectively weighted speech quality. SS, in a sense, is the trivial type of such VRC. This may be contrasted to variation in speech codec rate simply through switching the selected rate to an alternative rate. VRC can reduce average digital rate significantly through exploiting inherent variations in voice information content while maintaining required quality. However application of such VRC may not be readily possible in the context of all communication networks or systems. An application that has successfully utilized VRC is the CDMA (Code Division Multiple Access) wireless system. Due to CDMA distinct characteristics (in contrast to Time Division Multiple Access and other digital wireless systems), practical VRC""s such as IS-733 QCELP (Qualcomm PureVoice codec a.k.a. QCELP) and IS-127 Enhanced Variable Rate Coder (EVRC) were introduced. QCELP is also used in several Internet applications most notably JFAX(trademark), Apple(copyright), QuickTime(trademark), and Eudora(copyright). With the success of CDMA technology, on the road to the third generation wireless systems (3G or IMT-2000), VRC will likely be the dominant speech compression scheme in future wireless systems. A sub-committee of TIA (3GPP2-SWG) has already initiated the standardization of next generation VRC under the title of Selectable Mode Vocoder (SMV). Other than one mode of SMV, which is reserved for backward compatibility with EVRC, other modes promise to provide very low average rate with quality target close to G.723.1 at 6.3 kb/s as well as slightly higher average rate but with higher quality (same as IS-733). To demonstrate examples of coding efficiency provided by VRC it is shown that for a typical speech pattern, average rate of 6.8 and 4.7 for IS-733 QCELP and EVRC are possible. Constant rate codecs provide equivalent quality at around 10 and 8 kb/s respectively. SMV promises to reduce these compression figures.
ATM and IP (Internet Protocol) technologies have provided networking infrastructure alternatives for the turn of century communication systems. The background and prior art related to the network and system issues of this invention are mainly found in recent standardization efforts in ATM forum and ATM activities in ITU-T, most notably, ATM adaptation Trunking using AAL2 for narrowband service. Other related documents are ITU-T documents I.363.2 (adaptation layer of type 2), I.366.2 (service specific convergence sublayer), as well as ATM Forum document AF-VTOA-0113.000 (ATM trunking using AAL2).
These standardizations fulfill the urgent market need for an efficient transport mechanism to carry voice, voice-band data, circuit mode data, frame mode data, and fax traffic. The voice transport supported includes compressed voice and non-compressed voice together with SS and encompasses CES. Other related prior art includes U.S. Pat. Nos. 5,606,552; 5,481,544; and 5,870,397. These references and the above ITU and ATM Forum standards disclose fundamental means to efficiently transport voice and other data (single and multi-channel) over ATM. The scope of this prior art is however limited to SS and does not encompass VRC voice over ATM using enhancements to utilization of AAL2. AF-VTOA subcommittee recently initiated extensions of such voice trunking and transport over ATM LANs to newly emerged application areas. These applications, referred to as LLES, entail integrated voice and data access over ATM networks with connections to customer premises based on Digital Subscriber Line (xDSL), Hybrid Fiber Coax (HFC), or fixed wireless technologies. With the exception of SS, AAL2 and these activities however have been limited to constant rate voice compression.
VRC support is not part of conventional voice telephony that almost without exception has used TDM, which uses fixed dedicated bandwidth or constant bit rate for compressed speech. Voice telephony applications over ATM using AAL2 (e.g. AF-VOTA-0113.000) exploit only SS, a more primitive type of VRC. The proposed scheme in this invention takes advantage of popularity of AAL2 and provides a low cost development roadmap to allow efficient VRC voice over ATM using novel extensions to the AAL2 utilization. The AF-VOTA-0113.000 application domain, CES trunking, can be extended to newly emerged LLES narrowband and broadband access and transport applications. PBX/CO-to-CO (PBX=Private Branch Exchange) (CO=Central Office), PBX-to-PBX, xDSL, and (fixed) wireless access of VRC access and transport ATM are among potential applications. As AAL2 standard capabilities and compatibility are utilized and maintained, the invention herein remains cost effective.
In its simplest form, the invention uses AAL2 standard mechanism to define support for VRC and hence has no incompatibility bearing on existing systems supporting AAL2. A typical VRC profile encompasses options for all sub-rates within one or more VRC standard or proprietary variable rate codec (e.g. EVRC IS-127). The individual sub-rate (e.g. full rate, half rate, or one eight rate in IS-127) encoded audio format resembles any audio algorithm format listed in AAL2. The output of rate determination algorithm (RDA), commonly part of variable rate codec, is fed into present AAL2 inter-working function (IWF). The IWF in AAL2, which normally supports SS or multiple rates (as opposed to voice content VRC), is extended to accommodate VRC and thereafter automatically furnishes AAL2 capabilities with extensions to VRC. These capabilities, outlined in AF-VOTA-0113.000, include Multiplexer and Demultiplexer function to combine individual (VRC or otherwise) channels, narrowband signaling (Channel associated signaling or CAS and Common Channel Signaling or CCS), idle channel suppression, AAL2 SSCP (System Services Control Point) and CPS (Common Part Sublayer) functions, VCC (Virtual Channel Connection) management, et cetera. A simple mapping of frame number in a typical VRC speech codec into AAL2 sequence number is proposed. This allows for easy use of available error concealment techniques based on frame error, already part of VRC codec standards such as EVRC.
When using tandem-free-operation (TFO) of VRC-ATM, higher end-to-end voice quality and bandwidth efficiency can be provided. More advanced capabilities of the invention can provide TFO VRC across access and transport networks involving ATM. More complex system and network interworking functions in such cases may or may not be cost effective and practical and benefit tradeoffs need to be considered.